Referring to FIG. 1, in a signal acquisition system, a current sensor 11 outputs an alternating-current component of current, and therefore a canceling circuit 12 is generally arranged to acquire and cancel a direct-current component of the output current of the current sensor 11. However, limited setup time of the canceling circuit 12 may affect the wait time of the subsequent circuits, thereby lowering the efficiency of the entire signal acquisition system.
However, at the stage of sampling, when a first enable signal sh is high, and a first switch S1 is closed, a gate and a drain of a first P-type MOS transistor M1 are short-circuited. Therefore, the first P-type MOS transistor M1 is functionally equivalent to a resistance having a resistance of 1/gm1. gm1 is a transconductance of the first P-type MOS transistor M1. The equivalent resistor forms a time constant τ1=(C0+C1)/gm1 (formula 1), with a first capacitor C1 and a capacitor C0 of the current sensor. Therefore, when the transconductance gm1 of the first P-type MOS transistor M1 is small and the capacitance of the capacitor C0 of the current sensor is great, the time constant τ1 may be very great, such that the setup speed of the canceling circuit 12 is lower. In addition, to improve the setup speed of the canceling circuit 12, a general approach is to add a bias current source I1, such that the direct current flowing through the first P-type MOS transistor M1 is increased, and thus the transconductance gm1 of the first P-type MOS transistor M1 is increased, so as to finally reduce the time constant τ1 and improve the setup speed of the canceling circuit 12. However, a large amount of current noise may be introduced after the bias current is increased, which thereby affects the signal-to-noise ratio of the signal acquisition system.
Therefore, it may be a technical problem to be urgently solved in the related art as how to better implement current sampling and holding.